I haven’t solved it, but my meta-reasoning would begin with the fact that the puzzle was originally directed to people who cannot be assumed to know about HPMOR, or even HP. The actual puzzle has to use the text in some other way, and the puzzle is to find the actual puzzle. All those day numbers—what else might they mean? Is there exactly one named character in the text without a secret diary? Then the answer might involve writing theirs. Is there some pattern in who each character’s diary refers to on different days?
I haven’t solved it, but my meta-reasoning would begin with the fact that the puzzle was originally directed to people who cannot be assumed to know about HPMOR, or even HP. The actual puzzle has to use the text in some other way, and the puzzle is to find the actual puzzle. All those day numbers—what else might they mean? Is there exactly one named character in the text without a secret diary? Then the answer might involve writing theirs. Is there some pattern in who each character’s diary refers to on different days?
Oh, I just assumed the answer was the sequence of question-marked days.
Or maybe the answer is to just...(spoiler omitted, but consider gur ragenapr gb Zbevn).